5-Hydroxybenzo[b]thiophene-3-carboxylic acid derivatives of the general formula (I): ##STR2##
wherein R is hydrogen or a hydroxy-protecting group are important starting materials in the synthesis of pharmacologically active compounds. For example, a compound of the formula (I) is essential in the synthesis of benzothiophenecarboxamide derivatives of the general formula (VI): ##STR3##
wherein R is as defined above and X is hydrogen or alkyl. The benzothiophenecarboxamide derivatives are specific antagonists of PGD.sub.2 and known to be useful as a drug in the treatment of various diseases related to mast cell dysfunction caused by excessive production of PGD.sub.2, for example, systemic mastocytosis, disorder of systemic mast cell activation, tracheal contraction, asthma, allergic rhinitis, allergic conjunctivitis, urticaria, injury due to ischemic reperfusion, inflammation, and atopic dermatis (WO97/00853, PCT/JP97/04527 (WO98/25919)). Among compounds of the formula (VI), a compound wherein OR is 5-hydroxy and X is hydrogen (hereinafter, referred to as "Compound A") has especially high antagonistic effect on PGD.sub.2, showing an excellent anti-nasal occlusion activity, and is contemplated to be a promising drug for treating nasal occlusion.